The present invention relates to an apparatus for liquefying meltable material, especially melt adhesives, and the apparatus includes a supply container adapted to be filled with a material to be melted and having a perforated heatable bottom wall for melting the material and a collecting trough below the bottom wall for receiving the molten material passing through the perforations, and preferably a discharging device for transporting the molten material from the trough and feeding the same to an applying device.
Meltable materials, also called hot melts, find increasing use in the industry, for instance for coating substrates and also as temporary and/or permanent fusion adhesives.
The known hot melts are predominantly binary and tertiary mixtures of basic polymers, adhesive resins and waxes, softeners and fillers.
Depending on their use, one can differentiate between permanent and temeporary hot melts.
The permanent hot melts are predominantly so-called melt contact adhesives, also designated as pressure sensitive adhesives, since the adhesive layer retains also at room temperature a specific stickiness.
Temporary meltable materials and fusion adhesives have only in liquid form a predetermined minimum stickiness.
With the last group it is only possible to obtain a coating or cementing by changing the condition of the material from a liquid to a solid condition.
Hot melts and fusion adhesives are available in different form such as:
Temporary masses such as granulate, powder, slivers, pearls, strands, candles and so on, and
permanent masses such as blocks, strands and similar masses.
The basic polymers which are mainly used as hot melts can be divided in two main groups:
(a) Thermoplastic elastomers; and
(b) thermoplastic artificial resins, respectively plastics.
The thermoplastic elastomers include, among other materials, ethylene-vinylacetate-copolymers, ethylene-ethylacrylate-copolymers, polystyrole-butadien-polystyrene-block polymers, polystyrole-isoprene-polystyrene-block polymers, polyethylene, polypropylene, butylisobutyl- and isoprene-rubber types and ethylenepropylene rubber.
The thermoplastic artifical resins, respectively plastics, include polyvinyl acetate and the copolymers thereof, saturated polyester and copolyester, polyurethane, polyamides and copolyamides.
The hot melts and fusion adhesives are used for many different purposes, because they have many technical and commercial advantages. Thus the hot melts may be used to coat substrates to provide, for instance, a gas impermeable surface thereof. The temporary fusion adhesives are used, for instance, for cementing shoe and leather parts to each other, to glue carboards to each other, to glue the back of the books to the remainder thereof, to glue parts of furniture to each other, as well as during many other assembly and finishing operations.
The permanent adhesives are especially used for the production of adhesive tapes, adhesive foils, for self-sealing labels, and so on, that is at all occasions in which a permanent stickiness is required.
During the treatment of hot melts and fusion adhesives the stability of the material is one of the most essential parameters since these materials are during the treatment thereof in especially stimulated condition. In the molten liquid form these materials are move liable to be subjected to thermic and/or chemical-oxidizing reactions than in semisolid or solid condition. Such heat reaction may include a change in the color, especially a darkening of the material, a change of the viscosity thereof which is either reduced by depolymerization or increased by lattice-like polymerization, and deterioration of the rheological and adhesive characteristics of the material.
In order to prepare hot melts for use, the material is liquefied by heating in a liquefying apparatus and by a transporting device transported out of the apparatus. A heated transporting hose may, for instance, be connected to the apparatus and leading to a so-called application head.
An essential problematic of apparatus of the above-mentioned type consists in that the hot melts should usually not be heated above a predetermined temperature, respectively not be heated too long, since otherwise the mass will decompose. Since the material is usually fed in the form of granules from above into the supply container of the apparatus and passes continuously through the apparatus to thereby be liquefied, the amount of the hot melt which is maintained at relatively high temperature must be held relatively small. Furthermore, the amount of material in the supply container must be maintained at a sufficiently low temperature so that it will still remain in non-caking condition above the perforated bottom wall of the container. Correspondingly, the heated perforated bottom wall of the container should transfer the least possible amount of heat in upward direction, and further assure in the smallest possible vertical region the necessary heating of the mass from a temperature essentially below the softening temperature of the mass to a temperature which will produce liquefidation of the same. Finally, it is necessary that the temperature in the supply container should increase only from the top to the bottom thereof, while the temperature over the width of the container should be maintained substantially constant. This permits to approach the maximum permissible temperature more closely. Since the hot melt ought not to be excessively heated, the output of molten material per time unit is in known apparatus of this type relatively small.
A higher output could only be obtained if the heat transmitting surfaces of the perforated bottom wall of the container would be brought to a non-permissible high temperature, which in turn would reduce the quality of the adhesive.
In one known apparatus of this type, the perforated bottom wall of the supply container comprises a plate provided with vertically extending cylindrical bores which are heated by resistance heaters arranged in this plate. To control the temperature a temperature sensor is used, sensing the temperature of the perforated bottom wall and cooperating in the usual manner with a thermostat device which thus keeps the temperature of the perforated bottom wall constant. This known construction has a low efficiency, that is the output of liquefied material per time unit and unit of area of the bottom wall is relatively small. An additional essential disadvantage of this known apparatus is, that due to the relative high temperature prevailing in the collecting trough for heating the liquefied mass to the necessary end temperature, the temperature of the whole supply container, also at a considerable distance from the perforated bottom thereof, increases during operation of the device to such a high temperature that the mass in the container located considerably above the perforated bottom wall melts and bakes together in the upper region of the container, so as not to slide anymore downwardly in the same.
The heatable perforated bottom was originally provided in order to assure a preliquefying of the meltable mass at relatively low temperature, without excessively heating the mass in the container above the bottom wall and to subsequently thereto bring the liquefied material dropping into the collecting trough to the necessary temperature to transport the liquefied material to the application device.
This aim has not been or at least insufficiently accomplished with the constructions according to the prior art, that is either the mass of material in the supply container has been damaged by excessive heating, or the output of the apparatus of molten material at the proper temperature was too small. As mentioned before, in one of the known constructions the perforated bottom wall of the container mounted directly on a collecting trough is constructed as a planar, electrically heated plate provided with cylindrical vertical bores therethrough. In this construction a relatively large amount of heat rises from the perforated bottom wall upwardly and the heat transmission between the bottom wall and the mass in the container is bad. Nevertheless, the wall of the supply container and therewith the material therein is excessively heated in an undesired manner since the high temperature from the wall of the collecting trough is transferred directly into the wall of the supply container. Finally, the heating output is small since the perforated bottom has only a relatively small surface in contact with the mass passing therethrough.
In another known construction in which the perforated bottom is formed by a grate or rods of circular cross-section there is also not an improved output and no improved temperature distribution in the mass to be liquefied. In this second known construction the passage of the adhesive through the perforated bottom is relatively small, while the melting output is low. Nevertheless, a large amount of heat passes from the collecting trough maintained at high temperature into the wall of the supply container so that the material in the upper region of the latter will cake together. A further drawback of both known constructions is an unfavorable temperature distribution in the material in the supply container since the temperature of this material adjacent the wall of the container will be higher than in the center thereof.